In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol, or mixtures thereof.
An inkjet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer. There are generally two types of ink-receiving layers (IRL's). The first type of IRL comprises a non-porous coating of a polymer with a high capacity for swelling and absorbing ink by molecular diffusion. Cationic or anionic substances are typically added to the coating to serve as a dye fixing agent or mordant for the anionic or cationic dye, respectively. This coating is optically transparent and very smooth, leading to a high gloss “photo-grade” receiver. However, with this type of IRL, the ink is usually absorbed slowly into the IRL and the print is not instantaneously dry to the touch.
The second type of IRL comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra/inter-particle porosity. In this case, the printing liquid is substantially absorbed into the open pores of the IRL to obtain a print that is instantaneously dry to the touch.
Organic and/or inorganic particles in a porous layer form pores by the spacing between the particles. The binder is used to hold the particles together. However, to maintain a high pore volume, it is desirable that the amount of binder is limited. Too much binder would start to fill the pores between the particles or beads, which would reduce ink absorption. On the other hand, too little binder may reduce the integrity of the coating, thereby causing cracking.
As the quality and density of inkjet et printing increases, so does the amount of ink applied to the inkjet recording element (also referred to as the “receiver”). For this reason, it is important to provide sufficient void capacity in the medium to prevent puddling or coalescence and inter-color bleed. At the same time, print speeds are increasing in order to provide convenience to the user. Thus, not only is sufficient capacity required to accommodate the increased amount of ink, but in addition, the medium must be able to handle increasingly greater ink flux in terms of ink volume/unit area/unit time.
A porous ink jet recording element usually contains at least two layers: a lower layer, sometimes referred to as a base layer as the main sump for the liquids in the applied inkjet ink, and an upper layer, sometimes referred to as a gloss layer, often an image-receiving layer, coated in that order on a support. The layers may be sub-divided or additional layers may be coated between the support and the uppermost gloss layer. The layers may be coated on a resin coated or a non-resin coated support. The layers may be coated in one or more passes using known coating techniques such as roll coating, premetered coating (slot or extrusion coating, slide or cascade coating, or curtain coating) or air knife coating. When coating on a non-resin coated paper, in order to provide a smooth, glossy surface, special coating processes may be utilized, such as cast coating or film transfer coating. Calendering with pressure and optionally heat may also be used to increase gloss to some extent.
Recently, higher speed printing has been demanded of inkjet printers. A problem arises when multiple ink droplets are deposited in very close proximity in a short time. If the porosity of the receiver is not adequate, the drops will coalesce, severely degrading the image quality. The amount of binder in the coated layers is important in the performance of the ink-recording element. If too much binder is present, the porosity of the receiver is diminished resulting in coalescence, and if too little binder is present, unacceptable cracking is observed.
EP Patent Publication No. 1,464,511 to Bi et al. discloses a two-layer inkjet receiver on a resin-coated support. The bottom layer comprises a dispersion of fumed silica treated with aluminum chlorohydrate to transform the silica particles into a cationic form, as indicated by a zeta potential above +27 mv after treatment. The cationic silica particle dispersion was mixed with boric acid and poly(vinyl alcohol) to form a coating composition for the bottom layer. The coating composition for the top layer comprised a dispersion of cationic colloidal silica, glycerol, and a minor amount of coating aid. The top and bottom layers were cascade coated at the same time in one pass, that is, simultaneous coating is disclosed in context. The coating weight of the bottom layer was about 28 to 30 g/m2 and the top layer was 0.2 g/m2. However, there is a problem with this type of inkjet receiver in that image quality is reduced by coalescence when high ink levels are printed.
In the comparative example 4 of the above-mentioned EP Patent Publication No. 1,464,511 A2, a comparative inkjet recording element with a cationic fumed silica base layer and an anionic colloidal silica upper layer is made and tested.
US Patent Publication No. 2003/0224129 to Miyachi et al. discloses an inkjet recording element similar to the above-mentioned EP Patent Publication No. 1,464,511 in which a layer mainly containing cationic colloidal silica is over a base layer containing cationized anionic inorganic particles that can be fumed silica.
U.S. Pat. No. 7,015,270 to Scharfe et al. discloses an inkjet recording element comprising fumed silica and a cationic polymer in which the dispersion used to make the inkjet recording element has a positive zeta potential.
It is known to provide crosslinker, for a binder in an ink-receiving layer, by diffusion of the crosslinker into the layer. For example, Riou, et al., in U.S. Pat. No. 4,877,686, describe a recording sheet for inkjet printing and a process for its preparation. The coating composition comprises filler, such as an inorganic particle, and a polyhydroxylic polymeric binder, such as poly (vinyl alcohol). In the coating process, the PVA is gelled or coagulated by borax. The gelling agent may be deposited on the base material prior to the coating. Alternatively, the gelling agent can be incorporated in the coating composition, but must be temporarily deactivated. For example, boric acid may be used in the coating composition and activated by contact with a higher pH base layer. A drawback of this incorporated crosslinker process is that although the boric acid does not completely gel the PVA coating composition, viscosity increases may be expected, which may have a negative impact on coating quality throughout a coating event. The disclosure of Riou, et al., is mainly directed to providing more regular-shaped dots. High print density and gloss demanded of a photographic quality print are not addressed by Riou, et al.
Kuroyama, et al., in EP Patent Publication No. 493,100, disclose an inkjet recording paper comprising a substrate which is coated with boric acid or borate and an inkjet recording layer formed on the borax-coating and comprising synthetic silica and poly(vinyl alcohol). The silica may be wet-process silica, silica gel, or ultrafine silica obtained by a dry process. The exemplary silica materials are silica gels with high surface area, but with large secondary particle size of several microns or more. These materials do not provide a high gloss expected for a photo-quality print. Cationic polyelectrolytes may be added to improve water resistance, thus implying a composition compatible with cationic species.